The steam generators of pressurized water nuclear reactors comprise a bank of several thousand U-shaped tubes of a diameter close to two centimeters. Each tube comprises two rectilinear branches having a length close to ten meters and having their ends fixed in the tube plate of the steam generator, together with a substantially semicircular curved portion joining the two rectilinear branches at their tops.
The curved portions or bends of the tubes in a bank are not identical with one another, as the dimensions and radius of curvature of the bends depend on the position of the tubes in the tube bank. The bends of tubes lying in the central part of the bank have a much smaller radius of curvature than the bends of tubes situated towards the outside of the bank.
During the operation of a reactor, the tubes in the tube bank of a steam generator of a pressurized water nuclear reactor are in contact on their inner surface with the pressurized water circulating in the primary circuit and the vessel of the reactor, and on their outer surface with the feed-water which is introduced into the steam generator casing in order to be heated and vaporized by the heat supplied by the primary water.
The tubes of a steam generator in a pressurized water nuclear reactor are subject to corrosion during operation, and after a lengthy period of operation this may result in the cracking of these tubes in certain sensitive zones, such as the transition zone between the part of the tube which is deformed inside the tube plate during its fastening by crimping, and the undeformed part of the tube, or else the zone of the bends and their connection to the rectilinear branches. This severe corrosion and cracking in certain zones of the tube are connected with the presence in the tube wall of internal stresses resulting from the mechanical operations entailed in the shaping and installation of the tube bank, and of the external loads of thermal or mechanical origin undergone by the tube bank during the operation of the reactor.
Various methods have been proposed for the mechanical relief of stresses in tubes and have been used mainly for stress relieving in zones of the tubes situated inside or near the tube plate.
It has also been proposed to effect thermal stress relieving of the bands by introducing an electric resistance heating element into each of the tubes requiring treatment, starting from the water tank of the steam generator and disposing the heating element in the zone of the bend, the latter being heated for a determined period of time by supplying electric current to the heating resistor.
It is quite obvious that the operation of placing the heating element in position in the zone of the bend becomes increasingly difficult to achieve as the radius of curvature of the bend decreases. The heat treatment of the small bends of the bank is therefore a delicate operation which calls for the use of heating elements of special construction which can be placed in position in a part of the tube where the radius of curvature is small.
Heating elements of this kind may comprise a flexible mandrel on which an electric resistance wire is wound helically to form a winding whose outside diameter is smaller than the inside diameter of the tube to be treated.
The production of a flexible mandrel which can assume the curvature of the small bends and ensure satisfactory displacement and positioning of the heating element is extremely delicate and expensive. In addition, it is necessary to provide effective electrical and/or thermal insulation between the winding and the mandrel.
Furthermore, the tubes of the steam generator are often slightly ovalized in the zone of the bends, so that it is necessary to provide heating elements whose diameter is decidedly smaller than the inside diameter of the tube, and consequently the effectiveness and the output of the electric resistor are mediocre.
Conventional heating elements moreover do not enable a flushing gas to be injected into the heating zone in order to homogenize the treatment temperature along the bend.
Finally, in cases where steam generator tubes have to be treated over a long period of time, this treatment must be carried out in parallel on what may be a large number of tubes, in order to avoid a total treatment time incompatible with the maintenance program for the nuclear reactor during a stoppage. It is then necessary to have available a large number of heating elements, and if these elements are expensive, the investment required for effecting the stress relieving treatment is considerable.